ISPConfig 3 Installation - Postfix queue stuck

I've installed ISPConfig 3 over the past month. It works at first, but I then found with the following problem:
(conversation with 127.0.0.1[127.0.0.1] timed out while receiving the initial server greeting)

It always shown up in the queue.

I've Googled for this issue, and try almost all solutions in the first two pages, but it just failed again and again.